Crosslinking products of fluororubber have a variety of advantageous properties, but still have such disadvantages as a poor moldability at the time of vulcanization molding using a mold, and a high post-molding adhesiveness between the rubbers themselves. Thus, it is a usual practice to apply an external mold-releasing agent of such silicone series, fluoro-carbon series, etc. to the mold, surface or add an internal mold-releasing agent to unvulcanized rubber compounds. A vulcanization molding process without an external mold-releasing agent has been now desired particularly from the viewpoint of productivity.
Fluororubber has such a problem that, when the temperature is elevated at the time of the kneading of fluororubber in a kneader, a Banbury mixer, etc., the compounds present inside the kneader tend to undergo adhesion. In that case, application of an external mold-releasing agent is not so effective that the method of adding an internal mold-releasing agent to unvulcanized rubber compounds is usually recommended. In the case of using the internal mold-releasing agent, it is necessary for the internal mold-releasing agent to have not only improvements in the mold releasability and adhesivelessness at the time of kneading naturally, but also no adverse effects on normal state physical properties, compression set characteristics, and further on the heat resistance, chemical resistance, oil resistance, etc. inherent in the fluororubber.
It has been proposed to use low molecular weight polymers of tetra-fluoroethylene as a fairly satisfactory internal mold-releasing agent under these conditions. When the polymers are added in such a large proportion to thoroughly satisfy the mold releasability and adhesivelessness at the time of kneading in that case, there are such problems as increased hardness and restricted degree of freedom as to the compounding, etc., so that no satisfactory level has been attained yet.                Patent Literature 1: JP-B-56-40168        Patent Literature 2: JP-A-58-23846        Patent Literature 3: JP-A-59-41350        
Recently, short-time vulcanization has been keenly desired from the viewpoint of productivity, and it has been a usual practice to add a crosslinking promoter, an acid acceptor compound, etc. to accelerate the vulcanization rate. However, in the case of compression molding, not only such problems tend to appear as premature starting of vulcanization before the mold is fully filled with compounds, and occurrence of foaming due to insufficient vulcanization, but also such another problem is liable to occur as spontaneous vulcanization progress due to long-term preservation of compounds at room temperature. According to a method of elevating a molding temperature to accelerate the vulcanization rate, the viscosity of rubber is decreased with elevating temperature, and a gas volume is increased at the time of molding, resulting in a higher tendency for occurrence of foaming after the molding, particularly much foaming occurs in the case JIS A hardness is not more than 60.
When the JIS A hardness of crosslinking products of fluororubber exceeds 60, the sealability of HDD top cover gasket serving to seal at low specific pressure will be unsatisfactory. To make the JIS A hardness 60 or less, it is a usual practice to add liquid additives, but in the case of fluororubber addition of such liquid additives may impair the heat resistance, oil resistance, compression set characteristics, outgassing property, etc. inherent in the fluororubber. By lowering the crosslinking density, the hardness can be lowered without causing such problems, but to lower the crosslinking density, it is necessary to reduce the amount of the crosslinking agent or the acid accepter compound, resulting in a failure in acceleration of the vulcanization rate.
The conventional fluororubber of polyol crosslinking series needs addition of a large amount of such acid acceptor compound to attain the crosslinking, as magnesium oxide, calcium hydroxide, etc. Addition of a large amount of the acid acceptor compound leads to inevitable elevation of the hardness, which is not appropriate for HDD top cover gaskets serving to seal at low specific pressure. Metal ions of the acid acceptor compound such as magnesium ions, calcium ions, etc. have a possibility of contaminating the HDD, and thus it is desirable to use as small an amount of the acid acceptor compound as possible to reduce contamination with the metal ions. Particularly calcium compounds, when used, will cause serious HDD contamination.